1. Field of the Invention
The present invention relates to concentrates for alkaline two-component cleaners and methods of preparing and using same. More particularly, the invention relates to a method of making stable concentrates for alkaline two-component cleaners containing a silicic acid dispersion, tensides, stabilizers and optionally a high concentration of builder salts.
2. Description of Related Art
Alkaline cleaning agents are used for cleaning rigid materials such as, for example, work pieces made of metal or glass. These cleaning agents contain sodium hydroxide, orthophosphates, condensed phosphates, silicates, complexing agents, wetting agents and, optionally, corrosion inhibitors, anti-foaming agents and preservative. In general, products containing these components are commercially available as powders which are added to the cleaning bath. The concentration of the cleaning agent in the bath is maintained between 0.1 and 20% by weight, depending on the particular use and amount of dirt accumulation.
It is very important for purposes of cleaning efficiency to maintain the optimum concentration of the cleaner constituents in the cleaning solution. This may be determined by titration or conductivity measurements, for example. Conductive measuring devices may be combined with an automatic metering device for adding the cleaning powder in order to maintain a desired concentration of the cleaner.
Unfortunately, the continuous trouble-free metered addition of these powders is very difficult to achieve, especially when the powders contain hygroscopic salts. Furthermore, nonionic tensides are sensitive to solid sodium hydroxide and during extended storage periods the tensides tend to oxidize and form ether carboxylic acids. When this occurs, the strength of the cleaner deteriorates and, in spray applications, undesirable foaming occurs.
In addition to powdered cleaning agents, liquid alkaline cleaning concentrates are also known. U.S. Pat. No. 3,527,608 discloses a way of preventing the loss of polymer phosphates during extended storage of alkaline cleaning concentrates due to hydrolysis and the poor compatibility of the nonionic wetting agents, the anti-foaming agents and the polymer phosphates with the residual cleaning concentrate. U.S. Pat. No. 3,527,608 discloses supplementing these cleaning concentrates with a liquid acidic concentrate containing nonionic wetting agents and/or anti-foaming agents. The concentrates also contain water-soluble organic acids or acidic salts thereof which form complexes with substances causing water hardness. These concentrates, which typically exhibit acidic or neutral reactions, are made into liquid cleaners exhibiting alkaline reactions by the addition of excess alkali. Unfortunately, silicate-containing cleaners cannot be prepared using this procedure since the tensides used are incompatible with high concentrations of builder substances in the solution.
Published German patent application No. 32 46 080 discloses cleaners or cleaning solutions containing two components, one of the components being a sodium hydroxide solution, the other component containing the remaining active cleaning ingredients. The latter component solution may be adjusted to an acidic or neutral pH value, an acidic pH being advantageous in that all of the constituents, and more specifically the tensides, are particularly well soluble. The disclosed two-component cleaners, more specifically, are characterized in that the acidic or neutral component is an aqueous dispersion of silicic acid prepared from pyrogenic or precipitated silicic acid, which dispersion may optionally contain further components such as phosphoric acid, sulfuric acid, polyphosphates, tensides, stabilizers and/or preservatives.
Published German patent application No. 32 46 080 discloses preparing the silicic acid dispersions by precipitating silicic acid from water glass using sulfuric acid, following by washing and drying of the precipitate. However, this process is very expensive and difficult to carry out on an industrial scale. Moreover, the silicic acid obtained by this process has a relatively poor solubility in application such as a concentrate for a two-component cleaner. Poor solubility occurs when the SiO.sub.2 /Na.sub.2 O ratio in the cleaning bath is greater than 0.3:1. Another drawback in using precipitated and isolated silicic acid is, due to the limited solubilities of nonionic tensides, relatively strongly acidic concentrates must be prepared in order to ensure sufficient stabilization. This means that large amounts of alkali metal hydroxide must be added for neutralization or for achieving a preferred alkaline cleaning solution.